In recent years, from the social requirement of lower fuel consumption and less exhaust emission, an electric automobile and a hybrid automobile, each of which is mounted with an AC (alternate current) motor as a power source of a vehicle, is drawing attention. For example, in some of the hybrid automobiles, a DC power source made of a secondary battery or the like and an AC motor are connected to each other via an electric power conversion device constructed of an inverter and the like, and the DC voltage of the DC power source is converted into AC voltage by the inverter to thereby drive the AC motor.
In the control device of the AC motor mounted in the hybrid automobile or the electric automobile like this, there is known a control device of performing a feedback control on the basis of current sensed values of three phases, which are sensed by the current sensors. For example, a d-axis current and a q-axis current acquired by dq transforming the current sensed values of three phases are fed back to current commands.
Here, there is known the following technique (for example, patent document 1): one current sensor is provided for each phase of a three-phase AC motor; according to the Kirchhoff's law, the sum of currents of three phases to be supplied to the AC motor becomes zero; so that in the case where the sum of current sensed values of three phases does not become zero, it is determined that any one of current sensors is abnormal.
A technique for determining whether or not any one of current sensors is abnormal by monitoring the sum of sensed values of three phases will be described with reference to FIGS. 3A and 3B.
At the time of a normal control shown in FIG. 3A, the sum of a U-phase current iu, a V-phase current iv, and a W-phase current iw is always zero, as shown by an equation (1) based on the Kirchhoff's law.iu+iv+iw=0  (1)
Here, for example, as shown in FIG. 3B, it is assumed that an abnormality is caused in which a U-phase current becomes Ius offset with respect to iu on a plus side (positive side) by an error Δis and in which a V-phase current becomes Ivs offset with respect to iv by an error Δis, which is of the same amount as the error of the U phase, on a minus side (negative side). At this time, the sum of the current sensed values of three phases becomes 0 as shown by an equation (2), so that it is determined that current sensors are apparently normal.
                                                                        iu                +                ivs                +                iw                            =                            ⁢                                                (                                      iu                    +                                          Δ                      ⁢                                                                                          ⁢                      is                                                        )                                +                                  (                                      iv                    -                                          Δ                      ⁢                                                                                          ⁢                      is                                                        )                                +                iw                                                                                        =                            ⁢                              iu                +                ivs                +                iw                +                                  (                                                                                    +                                                                                                  ⁢                        Δ                                            ⁢                                                                                          ⁢                      is                                        -                                          Δ                      ⁢                                                                                          ⁢                      is                                                        )                                                                                                        =                            ⁢              0                                                          (        2        )            
Here, a similar situation can be brought about not only in the case where the sum of the current sensed values of three phases becomes zero but also in the case where the sum of the current sensed values of three phases is made “a value not larger than an abnormality threshold value” by the plus and minus (positive and negative) errors cancelling each other out. Hereinafter, this abnormality is referred to as “a plus/minus offset abnormality”.
Hence, in the case where a feedback control is performed by an operation in which an offset error is estimated on the premise of the Kirchhoff's Law, the offset error is never found during the feedback control. Furthermore, even in an open control in which the feedback control is not performed, the errors of two phases cancel each other out to thereby reduce a deviation from a normal value, which hence makes it difficult to detect a plus/minus (positive and negative) offset abnormality.
As a result, when the control of passing current through the AC motor is continuously performed without being aware of the occurrence of a plus/minus offset abnormality, an abnormal current is likely to be passed through the inverter and the windings of the AC motor, thereby being likely to fail the circuit and the elements or to make the AC motor output an abnormal torque. For example, in the case where an AC motor is mounted in an electric vehicle such as a hybrid automobile, torque variations, power variations, and vibrations of the vehicle are caused, which hence results in causing a reduction in drivability.
Furthermore, in the case where offset errors Δis of the U phase and the V phase are caused on the side of the same sign as shown in FIG. 4A, the magnitude of a resultant offset error Δis++ is not changed from an offset error Δis of only one phase. In contrast to this, in the case where offset errors Δis of the U phase and the V phase are caused on the plus side and on the minus side as shown in FIG. 4B, the magnitude of the resultant offset error Δis+− becomes √3 times the offset error Δis of only one phase.
In short, the plus/minus offset abnormality of two phases produces a larger effect on a system using an AC motor when compared with an offset abnormality of one phase in which an error is of same magnitude or an offset abnormality of the same sign of two phases in which an error is of same magnitude. Regardless of this, there is presented an problem that even if the sum of the current sensed values of three phases is monitored, a plus/minus offset abnormality cannot be detected.    [Patent document 1] Japanese Unexamined Patent Application Publication No. H06-253585